1. Field of the Invention
This invention relates to a method for soldering an electronic component on a substrate and a soldering structure of the electronic component.
2. Related Art
As a method for mounting an electronic component on a substrate, traditionally, soldering has been widely employed. The manner of soldering includes various techniques such as a method of forming a metallic bump serving as a connecting electrode formed on the electronic component by soldering and a method of “solder-precoating” of forming a solder layer on the electrode surface of the substrate. In recent years, from the point of view of environmental protection, in the above soldering, “lead-free solder” containing a least quantity of harmful lead has been adopted.
The lead-free solder is greatly different from the lead-series solder conventionally employed in their composition. Therefore, as regards the flux employed in a soldering process, the flux conventionally generally employed cannot be adopted as it is. Specifically, the conventional flux has an insufficient activating operation so that removal of an oxidation film on a solder surface is insufficient. Thus, it is difficult to assure good solder-wettability. For such solder having inferior solder-wettability, the flux having the composition in which metallic power of the metal with excellent solder-wettability such as silver is mixed has been proposed (for example, see JP-A-2000-31210). By using such flux, the solder molten in a re-flow process can be wet-spread along the surface of the metallic powder in the flux so that the solder molten can be led to an electrode which is a connecting target.
However, in the flux disclosed in the above Patent Reference, as the case may be, the following inconvenience has occurred according to the contents of the metallic powder. The mainstream in recent years is a non-cleaning technique in which cleaning for eliminating the flux component is not carried out after the soldering. Therefore, after the re-flow, the flux component remains as a residue deposited around a soldered portion. So the metallic powder contained in the flux also remains around the soldered portion.
At this time, if a large quantity of metallic powder remains, poor insulation due to migration may occur. In this case, if the contents of the metallic powder are reduced in order to prevent the poor insulation, the effect of leading the molten solder by the metallic powder in the re-flow process is lowered. As a result, the solder-connectivity was deteriorated. Particularly, in the soldering for manufacturing a semiconductor device by stacking a package component with semiconductor elements packaged in a resin substrate, a gap is likely to occur between the bump to be soldered and an electrode owing to warping of the resin substrate. As a result, poor connection due to solder-wettability occurred with high frequency. As described above, the conventional soldering method using the flux containing the metallic powder has a problem that it is difficult to combine the maintenance of solder-connectivity and assurance of insulation.